Wind turbines typically include a rotor with large blades driven by the wind. The blades convert the kinetic energy of the wind into rotational mechanical energy. The mechanical energy is typically transferred via drive train to a generator, which then converts the energy into electrical power.
Most modern wind turbines control power output by pitching the blades relative to the wind. Each blade is mounted to a hub by a respective blade bearing that allows relative movement between the blade and the hub. The blades are each rotated about its longitudinal axis by a pitch system that includes one or more electrical drives (e.g., electrical motors) or hydraulic drives (e.g., hydraulic actuators). Although a single drive may be used to collectively pitch the blades, the pitch systems in most modern, multi-megawatt wind turbines include separate drives for pitching each blade individually.
There are advantages and disadvantages associated with both hydraulic pitch systems and electrical pitch systems. Regardless of which type of system is used, the primary objective is to quickly and accurately control the rotation of the blade about its longitudinal axis. Most of the rotation occurs over a narrow range during power production. Small, cyclical movements characterize this rotation. However, there are times when a blade must be quickly pitched over a much larger range, such as when the blades must be pitched to a “stop” position. Optimizing a pitch system for both of these situations presents a challenge. Moreover, to ensure that pitching to a stop position can occur when there is a loss of power, pitch systems must include a back-up power supply. This presents a further design challenge no matter which type of pitch system is used.